MKAbeFook for Facebook. Uses the WebKit browser with asynchronous requests, photo uploads, and much more. It can call any API method and returns the result in standard Cocoa types. It is distributed under a BSD style license.
When it comes to map mashups there's no doubt that Google Maps leads the way. On ProgrammableWeb we list 1,964 Google Maps mashups, almost four times as many as any other API. This popularity makes utilities built on top of Google Maps especially interesting, such as this contextual menu control, which pops up a menu when the user right clicks.
Parse, a leading backend-as-a-Service mobile SDK provider, has launched a new service called Cloud Code making it possible for app developers to add custom backend code that is easily accessible by any client (iOS, Android, HTML5, or via the REST API) without servers.